Energy conservation and pollution control efforts are driving development and implementation of propulsion systems for marine vessels and for other heavy duty, variable demand applications that reduce both fuel consumption and harmful emissions. Diesel-electric propulsion has been used on submarines and surface ships for many years. In general, a diesel engine is coupled to an electric generator, which supplies power to an electric motor that is mechanically coupled to a propeller.
Various types of combined cycle propulsion systems that utilize two or more distinct power sources for propulsion, either together or separately, have also been implemented. For example, a CODOG (COmbined Diesel Or Gas) arrangement provides propulsion either by diesel engines or gas turbine engines, but not both simultaneously. In contrast, a CODAG (COmbined Diesel And Gas) arrangement would allow both types of propulsion at the same time, as well as independently. Combined cycle arrangements have been used with various combinations of diesel, diesel-electric, gas turbine and steam turbine propulsion. The Royal New Zealand Navy's multi-role vessel HMNZS Canterbury has a Combined Diesel Electric and Diesel (CODLAD) propulsion system.
U.S. Pat. No. 7,207,852 discloses a ship propulsion system in which a mechanical drive shaft extends through the hull and is driven by either a prime mover located within the ship or by an electric motor in a housing external to the ship through which the mechanical drive shaft extends. This arrangement provides the benefits of combined cycle operation without having to accommodate electric motors within limited interior space of a ship. A similar type of electric pod propulsion unit has been combined with a conventional diesel propulsion system in Ueda, N. and Numaguchi, Hajime, The First Hybrid CRP-POD Driven Fast ROPAX Ferry in the World, Translated from Journal of the JIME Vol. 40, No. 2, 2005.
Notwithstanding these efforts to provide combined propulsion systems for marine vessel applications, the combined systems have shown disappointing results, particularly in terms of overall fuel and performance efficiency, emissions reduction, versatility, and ability to respond quickly and efficiently to a range of propulsion demand situations.